Why do airplane transponders have an 'off switch?'

WASHINGTON (AP) -- Ever since Malaysia Airlines Flight 370 disappeared, a fascinated public has asked: Why can somebody in the cockpit shut off the transponder?

It turns out there are several legitimate reasons why a pilot might want to shut off this key form of communication that allows air traffic controllers to identify and track airplanes.

Authorities believe that Flight 370's transponder was intentionally shut off, delaying search and rescue efforts and helping to conceal the plane's location - a mystery unsolved more than 10 days after the Boeing 777 vanished.

It's rare for a pilot to turn off a transponder during flight, but occasionally there is cause.

- The device could have an electrical short or catch on fire. Pilots would want to shut it down rather than risk a fire spreading to the rest of the cockpit or airplane.

- Pilots used to routinely turn off transponders on the ground at airports so as not to overwhelm air traffic controllers with so many signals in one location. That is increasingly less the case as pilots now use "moving map" displays that take the transponder data and show them the location of other planes on the ground, helping guide them around airports without mishaps.

"As long as there are pilots, they'll be able to switch off systems," said Andrew Thomas, editor-in-chief of the Journal of Transportation Security.

Airplanes have two transponders. There are two knobs in the cockpit - one on the right, the other on the left - that control one or the other. When one transponder is on, the other is normally in standby mode.

To turn off a transponder, a pilot turns a knob with multiple positions and selects the "off" setting. The second transponder doesn't automatically activate if the first one is shut down - a knob would also have to be turned. In this case, it appears one transponder was turned off, and the second not activated.

Ross Aimer, a retired United Airlines pilot and former 777 instructor, said it is possible that one pilot could reach up and turn off the transponder without the other pilot seeing it, say if one was looking away or distracted.

If the plane was in contact with an air traffic controller, the controller would alert the pilots that the transponder signal had been lost. But, Aimer - now head of Aero Consulting Experts - said, if they were not in contact with an air traffic controller, a pilot might miss it if the other shut down the transponder.

In the case of the missing Malaysian plane, even pilots are a bit puzzled by somebody turning off the transponder.

John Gadzinski, a Boeing 737 captain, said that among fellow pilots "there is a raised eyebrow like Spock on `Star Trek' - you just sit there and go, `why would anybody do that?'"

Comments

Why can't a transponder recognize the altitude and stop sending a signal while on the ground. Second with technology as great as it is, why isn't there a second, third, Fourth or even a fifth transponder which communicates with satellites when ground radar is out of range? And these can not be turned off. These would only talk to satellites not ground radar. This way it would not confuse ground radar. There can be multiple of these transponders that talk to satellites. Some would tell how well the engines are running, direction heading, altitude and so on. There should never be a way to turn these devices off in flight or on the ground. Even in the event of a disaster these devices would still communicate their location and would keep talking for approximately 30 days as do the transponders of today. Lastly, built as strong as possible to resist a blast, fire etc. This is just my suggestion. Techs try and work on this idea.

It always takes an event to move forward with aviation safety. I foresee changes in ELT's or the addition of another electronic fail-safe device that could be inhibited, if need exists, by the radar/computer systems of air traffic control.

Someone knows something they are not releasing to the media. They must know where this plane is. Only so many places it could be; and we can see that detail from satellites. If a plane is off course so far like that; it is assumed it is a terrorist attack and would be shot down. Also; it may be covered up like TWA flt 800. We may never hear the truth about what happend with this plane.

There should be some other device for tracking the plane that no one can touch just like the black boxes! and a satellite system that covers remote areas of the oceans to be able to keep track of all vessels whether in the air or the water!

If we are capable of finding galaxies, planets and stars in distant outerspace, or measuring the ocean depths for earthquakes, and we seem to have the means to home in on and track microchips implanted in wildlife...it stands to reason that it's long overdue to imbed a fresh microchip on every plane out of reach of any human intervention in the air.

It would seem logical that having the "black boxes" or ELT (emergency locator transmitter) be interrogateable might be one solution. Ask the plane where it is instead of depending on the acceleration/deceleration sensors initiating the ELT signal.

I can tell you many many modifications are installed after production oof an aircraft.. As to the cost just bear in mind each time an airline tacks on more and more Items to your fare,things that were once included.

In all probability they have on -off switches, circuit breakers and electrical busses that could be isolated. They are turned off and on as part of normal checklists -On before flying -Off after landing.The capability to turn off in flight is part of abnormal checklists concerning -electrical fire and overheat and /or smoke in cockpit. Normally they are required to be left on while in air.

Be a former Pilot I have seen may times when crossing an area that I went off radar and was not seen for 30 minutes are so to pop up on another radar to hear glade to have you back but what dose not make any since to me is that it just disappeared now I am not trying to be funny at all for I fear for all on board and my Prayers go out to them and there families but a number od years ago on a flight of mine we saw something when we said this the tower said say again and my Co-Pilot says we have something a 3 0clock but it a second it was gone, They put us threw so much crap over this and a month later I am getting ready to depart Chicago and something comes over the air port and it just sat there it shut down the hole air port it must have been major for the us sent out 2 fighters and this thing was seen by the fighter pilots. So my thing is this did it go down or did it just taking by something. I just really prays they find them.

Can anyone explain to me why the plane does'nt have a GPS where a satellite can locate it, how about todays phones, iphones, they all have GPS technology by satellite, how come the satellites can't track the GPSs, you can track a dog with a GPS anywhere in the world...Well?